The present invention relates to an apparatus for counter-current contact between two at least partially immiscible fluids with different specific gravities.
More in particular, the invention relates to such an apparatus, suitable for example for the extraction of one or more components from a first fluid phase by a second fluid phase, which apparatus has a plurality of superposed mixing compartments with rotatable agitator means therein and separated from each other by perforated separating walls. In such an apparatus the specifically heavier fluid phase is fed to, and the lighter fluid phase is discharged from the uppermost mixing compartment and the lighter fluid phase if fed to and the heavier liquid phase is discharged from the lowermost mixing compartment. Such an apparatus may be considered as a series of consecutive mixers, through which the fluid phases are counter-currently passed. In order to suppress back-mixing of fluid to a preceding compartment perforated walls are installed between the compartments.
An example of the above type of fluid/fluid contact apparatus is known from British patent specification No. 1,223,902.
In fluid contact systems it is generally recommended to have the fluid phase with the larger volumetric flowrate as the dispersed phase, since this results in a high dispersed phase hold-up and therefore in a larger interfacial area, thus larger separating efficiency as compared to systems in which the fluid with the smaller volumetric flowrate forms the dispersed phase.
However, if the ratio of the respective volumetric flowrates is rather large, for example 4:1 or even more, stable operation with the fluid having the larger volumetric flowrate as dispersed phase will only be possible at relatively low loads, and therefore low hold-up of dispersed phase. At higher throughputs the hold-up of dispersed phase does increase, but may be accompanied with phase inversion, which should be avoided in commercial operation of fluid contact systems. If phase inversion occurs, coalescence and settling of dispersed phase have to take place at the other end of the contact apparatus than designed, which will result in a completely unsatisfactory unstable operation.
If the ratio of the flowrates is large, stable operation of the contact apparatus requires to have the fluid with the smaller volumetric flowrate as the dispersed phase but, as stated above, this will result in a relatively low hold-up of dispersed phase and hence in a relatively small interfacial area and consequently a relatively poor contact efficiency. One solution for this problem might be to carry out the contact operation in so called mixer-settlers, and to recirculate dispersed phase from the settler to the mixer. This solution however is for practical reasons unattractive if a large number, say three or more, of socalled theoretical stages is required.
The object of the present invention is to provide an apparatus for counter-current contact of fluids with a high efficiency, which apparatus is particularly suitable for contacting fluids with a large ratio of their respective volumetric flowrates.